1. Field of the Invention
This invention relates to the real time, three dimensional imaging of the anatomy using a single pulse of ultrasound or a reduced number of pulses of ultrasound--ie. a real time 3D medical ultrasound machine.
2. Description of the Prior Art
The above mentioned patents and patent applications (ie. mentioned in CROSS-REFERENCES TO RELATED APPLICATIONS) discuss 3D real time imaging and imaging using ellipsoidal backprojection. These describe, in detail, ellipsoidal backprojection using small transmitters which emit nearly spherical waves.
This patent application describes how to use large extended transmitters with ellipsoidal backprojection; and describes also how, when the large extended transmitter is carried to its logical extreme, ellipsoidal backprojection becomes paraboloidal backprojection.
An annular array can be used, in defocused mode, to synthesize a large extended transmitters of differing radius. Thus a very large number of transmitters can be synthesized using an annular array containing only a few actual transmitter elements.
The use of extended transmitters has a very significant advantage: It yields an increase in signal in signal to noise ratio due to a) the increased active area of the transmitter, b) the partial confinement of radiated energy to the imaged volume, and c) the decreased loss due to spherical spreading.
Extended transmitters can also have disadvantages: Residual edge waves cause come clutter in the echoes and, in the case of paraboloidal backprojection, only one transmitter can be used thus making unfeasible very large numbers of backprojections per reconstructed point.
Patent application Ser. No. 07/387,615 filed Jul. 28, 1989 describes filtered ellipsoidal backprojection with one or many small transmitters. The present patent application describes inverse triplet filtered ellipsoidal and paraboloidal backprojection with one or many large extended spherically curved transmitters. The inverse triplet filter allows real time, high resolution, low sidelobe level, three dimensional imaging with almost any type of finite duration transmitted pulse. In particular, pulse sequences that can be compressed upon reception can be used, with resulting improved signal to noise ratio, for example see "Ultra-wideband Radar" CRC Press, 1991. Also conventional 11/2 to 31/2 cycle damped sinusoidal ultrasound pulses can be used.
The following is a discussion of prior art, much of which discussion is common to the applicant's previous patents and patent applications:
The reconstruction technique described in U.S. Pat. No. 4,706,498 is essentially the backprojection of the echo samples over ellipsoids of revolution as will be more fully described in this application. The backprojections may be weighted as a function of the reconstruction point position to compensate for transmitter or receiver radiation patterns and other phenomena.
This imaging system can be implemented with nearly all commonly used types of transmitted pulses. The transmitted pulses that the imaging system may use also includes pulses with peaked autocorrelation functions that have a very small value except when the shift variable is near zero. These types of pulses will be termed "non interfering" or "interference free" for purposes of this application as there is little constructive and destructive interference and therefore strong grating lobes will not be formed when using a sparse array. A wideband white noise pulse is an extreme example. These types of pulses also can propagate relatively uniformly through a wide solid angle. Further discussion of these types of pulses may be found in "Random Data:Analysis and Measurement Procedures" by Bendat and Piersol.
The imaging system can also function with a class of pulses which will be termed "low interference" for purposes of this application. This type of pulse has relatively low constructive and destructive interference effects. The autocorrelation function is relatively peaked around zero with relatively low amplitude oscillations as the shift variable takes on non zero values and therefore high amplitude grating lobes will not be formed when using a sparse array.
Periodic, oscillating, "interfering" pulses of a particular class may also be used for imaging if additional echo processing occurs before image reconstruction (such as echo time history convolution with a matched filter impulse response) or without additional processing if some image degradation is allowable. The pulses must be of short enough duration to allow adequate lateral and range resolution. Thus, a pulse of several sinusoidal cycles may be used if the total pulse duration, or length, is of the same order as the required resolution. These types of pulses will be termed "short duration interfering" pulses. Alternately some types of long duration pluses can be compressed upon reception, for example chirped pulses. These, sometimes interfering type, pulses also work well with ellipsoidal and paraboloidal backprojection.
Patent application Ser. No. 07/106,577, which is a continuation in part of U.S. Pat. No. 06/858,696 further describes the Ellipsoidal Backprojection image reconstruction technique which is used for image reconstruction in U.S. Pat. No. 06/858,577.
Ellipsoidal projections provide a general theory for the analysis of active imaging systems, see "Active Imaging Greens Function" in Acoustical Imaging vol. 19, Imaging Press. Ellipsoidal Backprojection is a method for the active imaging of a three dimensional volume using a single transmitted pulse or greatly reduced number of transmitted pulses and is discussed in detail in the previously mentioned patents and patent applications. Referring to FIG. 1, a short pulse of energy is transmitted which radiates outward, as an expanding sphere, through a wide solid angle. Echoes are received by a sparse array of receiver elements and, typically, then digitized into echo samples. These samples (which may be filtered first) are then backprojected over ellipsoids through the image by one means or another. The results constitute, basically, the reconstructed image, although additional processing steps may be implemented.
Ellipsoidal Backprojection is a linear image reconstruction method, although the point spread function varies with location of the reconstruction point. The point spread function is the image of a point object as reconstructed by the imaging system. In a linear imaging system, the final, reconstructed image is, essentially, the convolution of the point spread function with the original object to be imaged. This is well known and is described in: Introduction to Fourier Optics--Goodman; Linear Systems, Fourier Transforms, and Optics--Gaskill; or the Fourier Transform and its Application to Optics--Duffiuex.
The point spread function determines the imaging capabilities of a linear imaging system. Ellipsoidal Backprojection alone can yield an adequate point spread function for some applications, however, a particular type of linear filter, the "inverse triplet filter" (discussed in detail here), may be applied to the echo time histories, before backprojection, which greatly improves the resolution while substantially reducing the sidelobe levels. The resulting imaging system is still linear.